Network, generic

U0027 Low Speed CAN Communication Bus (-) Shorted to Bus (+)

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0027 describes the two wires of a low-speed CAN, (+) and (-), touching each other. On a high-speed bus such a short ends all traffic, but the fault-tolerant layer of ISO 11898-3 is built to ride it out. NXP's TJA1055 counts it as failure 7: the receive signal first sticks at dominant, then within about 0.3 to 1.6 ms the transceiver turns off its CAN-Low driver, leaves that wire with only a weak pull-up and carries on in single-wire mode. Comfort functions usually still work, though with less margin against interference, and differential operation resumes by itself once the short is cleared. No manufacturer documents U0027 publicly; a single-wire bus like GM's low-speed GMLAN cannot have this fault at all. Squeezed harness sections and wet splices are the likely spots; compare the high-speed sibling U0009.

Likely causes

Harness crushed so that both bus wires make contact
Water in a splice or connector bridging the two wires
Damaged transceiver bridging its two bus pins

Symptoms

  • Comfort network keeps running, sometimes with dropouts
  • Higher sensitivity to electrical interference
  • Code stored in several body modules at once

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–700
Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

low speed can wiringcan high wirecan low wire

Related codes

U0009U0018U0019U0021U0024

References: nxp.com, law.cornell.edu, saabwisonline.com.